U.S. patent application number 13/555794 was filed with the patent office on 2013-01-24 for photovoltaic cell enhancement through uvo treatment.
This patent application is currently assigned to UNIVERSITY OF FLORIDA RESEARCH FOUNDATION, INC.. The applicant listed for this patent is Song Chen, John R. Reynolds, Cephas Small, Franky So. Invention is credited to Song Chen, John R. Reynolds, Cephas Small, Franky So.
Application Number | 20130019937 13/555794 |
Document ID | / |
Family ID | 47554923 |
Filed Date | 2013-01-24 |
United States Patent
Application |
20130019937 |
Kind Code |
A1 |
So; Franky ; et al. |
January 24, 2013 |
PHOTOVOLTAIC CELL ENHANCEMENT THROUGH UVO TREATMENT
Abstract
Photovoltaic cells, methods of fabricating photovoltaic cells,
and methods of using photovoltaic cells to capture light energy are
provided. A photovoltaic cell can include an electron transporting
layer, a photoactive layer, and a hole transporting layer. The
electron transporting layer can be ultraviolet ozone treated. The
photovoltaic cell can have an inverted configuration.
Inventors: |
So; Franky; (Gainesville,
FL) ; Reynolds; John R.; (Dunwoody, GA) ;
Chen; Song; (Gainesville, FL) ; Small; Cephas;
(Gainesville, FL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
So; Franky
Reynolds; John R.
Chen; Song
Small; Cephas |
Gainesville
Dunwoody
Gainesville
Gainesville |
FL
GA
FL
FL |
US
US
US
US |
|
|
Assignee: |
UNIVERSITY OF FLORIDA RESEARCH
FOUNDATION, INC.
Gainesville
FL
|
Family ID: |
47554923 |
Appl. No.: |
13/555794 |
Filed: |
July 23, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61510804 |
Jul 22, 2011 |
|
|
|
Current U.S.
Class: |
136/256 ;
257/E31.026; 438/85 |
Current CPC
Class: |
H01L 2251/308 20130101;
H01L 51/442 20130101; Y02E 10/549 20130101; H01L 51/4253
20130101 |
Class at
Publication: |
136/256 ; 438/85;
257/E31.026 |
International
Class: |
H01L 31/0216 20060101
H01L031/0216; H01L 31/18 20060101 H01L031/18 |
Goverment Interests
[0002] The subject invention was made with government support under
the Office of Naval Research, Contract No. N000141110245; the
Department of Energy Basic Energy Sciences, Contract No.
DE-FG0207ER46464; and the Air Force Office of Scientific Research,
Contract No. FA9550-09-1-0320. The government has certain rights to
this invention.
Claims
1. A photovoltaic cell, comprising: a first electrode; an electron
transporting layer (ETL); a photoactive layer; a hole transporting
layer (H TL); and a second electrode, wherein the ETL is
ultraviolet-ozone (UVO)-treated.
2. The photovoltaic cell according to claim 1, wherein the
photovoltaic cell has an inverted configuration, such that the
first electrode is a cathode, the ETL is on the first electrode,
and the second electrode is an anode.
3. The photovoltaic cell according to claim 1, wherein the ETL
comprises metal oxide nanoparticles.
4. The photovoltaic cell according to claim 1, wherein the ETL is a
zinc oxide (ZnO)-polyvinylpyrollidone (PVP) nanocomposite film.
5. The photovoltaic cell according to claim 1, wherein the first
electrode is a cathode and comprises a transparent conductive
oxide, and wherein the second electrode is an anode and comprises a
metal.
6. The photovoltaic cell according to claim 1, wherein the
photoactive layer comprises a polymer and a fullerene.
7. The photovoltaic cell according to claim 6, wherein the
fullerene is (6,6)-phenyl-C71-butyric acid methyl ester
(PC71BM).
8. The photovoltaic cell according to claim 7, wherein the polymer
is poly(dithienogermole)-thienopyrrolodione (PDTG-TPD),
poly(distannyl-dithienogermole)-thienopyrrolodione, or
poly(dithienosilole)-thienopyrrolodione (PDTS-TPD),
9. The photovoltaic cell according to claim 1, wherein the
photoactive layer comprises copper indium gallium (di)selenide
(CIGS).
10. The photovoltaic cell according to claim 1, wherein the HTL
comprises molybdenum oxide.
11. The photovoltaic cell according to claim 1, wherein the first
electrode comprises indium tin oxide (ITO), and wherein the second
electrode comprises silver or aluminum.
12. The photovoltaic cell according to claim 1, wherein the second
electrode is transparent to at least a portion of visible
light.
13. The photovoltaic cell according to claim 2, wherein the ETL is
a ZnO--PVP nanocomposite film, wherein the first electrode
comprises ITO, wherein the second electrode comprises silver or
aluminum, wherein the photoactive layer comprises a polymer and a
fullerene, wherein the fullerene is PC71BM, wherein the polymer is
PDTG-TPD, poly(distannyl-dithienogermole)-thienopyrrolodione, or
PDTS-TPD, and wherein the HTL comprises molybdenum oxide.
14. A method of fabricating a photovoltaic cell, comprising:
forming an ETL on a first electrode; performing UVO treatment on
the ETL to provide a UVO-treated ETL; forming a photoactive layer
on the UVO-treated ETL; forming an HTL on the photoactive layer;
and forming a second electrode on the HTL.
15. The method according to claim 14, wherein forming the ETL
comprises sputtering or solution processing a metal oxide.
16. The method according to claim 14, wherein the ETL is a ZnO-PVP
nanocomposite film, wherein forming the ETL comprises baking a
coated mixture of zinc acetate and PVP in a solvent, and wherein
the solvent is ethanol, ethanolamine, or a mixture of ethanol and
ethanolamine.
17. The method according to claim 14, wherein forming the HTL
comprises thermal evaporation or solution processing.
18. The method according to claim 14, wherein forming the
photoactive layer comprises sputtering, co-evaporation, or solution
processing.
19. The method according to claim 14, wherein the first electrode
is a cathode, and wherein the second electrode is an anode.
20. A method of capturing light energy from light, comprising:
providing a photovoltaic cell such that the light is incident upon
the photovoltaic cell, wherein the photovoltaic cell comprises: a
first electrode; an ETL; a photoactive layer; an HTL; and a second
electrode, wherein the ETL is UVO-treated.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application claims the benefit of U.S.
Provisional Application Ser. No. 61/510,804, filed Jul. 22, 2011,
which is hereby incorporated by reference in its entirety,
including any figures, tables, or drawings.
BACKGROUND OF THE INVENTION
[0003] Photovoltaic cells are considered an important source of
renewable energy for helping to solve the world's energy shortage
today. Various photovoltaic cell technologies have been developed,
and polymer bulk heterojunction (BHJ) photovoltaic cells have
received attention because of their compatibility with large-scale
roll-to-roll (R2R) processing. These photovoltaic cells typically
exhibit low power conversion efficiencies of less than 3%.
BRIEF SUMMARY OF THE INVENTION
[0004] Embodiments of the subject invention are drawn to novel and
advantageous photovoltaic cells, as well as methods of
manufacturing and methods of using such photovoltaic cells. A
photovoltaic cell can have an inverted configuration and can
include at least one layer that has been treated with an
ultraviolet ozone (UVO) treatment. The UVO-treated layer can be an
electron transporting layer (ETL), and the ETL can be, for example,
a metal oxide or a metal oxide-polymer nanocomposite layer.
[0005] In an embodiment, a photovoltaic cell can include: a first
electrode; an ETL; a photoactive layer; a hole transporting layer
(HTL); and a second electrode. The ETL can be UVO-treated.
[0006] In another embodiment, a method of fabricating a
photovoltaic cell can include: forming an ETL on a first electrode;
performing UVO treatment on the ETL to provide a UVO-treated ETL;
forming a photoactive layer on the UVO-treated ETL; forming an HTL
on the photoactive layer; and forming a second electrode on the
HTL.
[0007] In another embodiment, a method of capturing light energy
from light can include: providing a photovoltaic cell such that the
light is incident upon the photovoltaic cell. The photovoltaic cell
can be as described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 shows a cross-sectional view of a photovoltaic cell
according to an embodiment of the subject invention.
[0009] FIG. 2a shows current-voltage characteristics for
photovoltaic cells according to embodiments of the subject
invention.
[0010] FIG. 2b shows external quantum efficiency spectra for
photovoltaic cells according to embodiments of the subject
invention.
[0011] FIG. 3 shows photo J-V characteristics for a photovoltaic
cell according to an embodiment of the subject invention.
[0012] FIGS. 4a and 4b show 3-D surface topography images for films
of photovoltaic cells according to embodiments of the subject
invention.
[0013] FIGS. 4c and 4d show phase images for films of photovoltaic
cells according to embodiments of the subject invention.
[0014] FIGS. 4e and 4f show schematics for films of photovoltaic
cells according to embodiments of the subject invention.
[0015] FIGS. 5a-5c show X-ray photoemission spectra (XPS) for films
of photovoltaic cells according to embodiments of the subject
invention.
[0016] FIG. 6a shows a plot of current density vs. time for a
photovoltaic cell according to an embodiment of the subject
invention.
[0017] FIG. 6b shows a plot of fill factor vs. time for a
photovoltaic cell according to an embodiment of the subject
invention.
[0018] FIG. 6c shows a plot of power conversion efficiency vs. time
for a photovoltaic cell according to an embodiment of the subject
invention.
[0019] FIGS. 7a and 7b show 5 .mu.m-scale phase images for films of
photovoltaic cells according to embodiments of the subject
invention.
[0020] FIG. 8 shows UV-visible-NIR transmission spectra for films
of photovoltaic cells according to embodiments of the subject
invention.
DETAILED DISCLOSURE OF THE INVENTION
[0021] When the terms "on" or "over" are used herein, when
referring to layers, regions, patterns, or structures, it is
understood that the layer, region, pattern or structure can be
directly on another layer or structure, or intervening layers,
regions, patterns, or structures may also be present. When the
terms "under" or "below" are used herein, when referring to layers,
regions, patterns, or structures, it is understood that the layer,
region, pattern or structure can be directly under the other layer
or structure, or intervening layers, regions, patterns, or
structures may also be present. When the term "directly on" is used
herein, when referring to layers, regions, patterns, or structures,
it is understood that the layer, region, pattern or structure is
directly on another layer or structure, such that no intervening
layers, regions, patterns, or structures are present.
[0022] When the term "about" is used herein, in conjunction with a
numerical value, it is understood that the value can be in a range
of 95% of the value to 105% of the value, i.e. the value can be
+/-5% of the stated value. For example, "about 1 kg" means from
0.95 kg to 1.05 kg.
[0023] Embodiments of the subject invention are drawn to novel and
advantageous photovoltaic cells, as well as methods of
manufacturing and methods of using such photovoltaic cells. A
photovoltaic cell can have an inverted configuration and can
include at least one layer that has been treated with an
ultraviolet ozone (UVO) treatment. The UVO-treated layer can be an
electron transporting layer (ETL), and the ETL can be, for example,
a metal oxide-polymer nanocomposite layer.
[0024] Referring to FIG. 1, in an embodiment, a photovoltaic cell
100 can include a first electrode 20, an ETL 30, a photoactive
layer 40, a hole transporting layer (HTL) 50, and a second
electrode 60. The first electrode 20 can be a cathode, and the
second electrode 60 can be an anode, though embodiments are not
limited thereto. In a particular embodiment, the first electrode 20
can be a bottom cathode, and the second electrode 60 can be a top
anode, such that when the photovoltaic cell is in use, the
photovoltaic cell has an inverted configuration. Light can enter
the photovoltaic cell 100 from, for example, the bottom of FIG. 1,
such that it goes through the first electrode 20, though
embodiments are not limited thereto. Though certain materials are
listed in FIG. 1, embodiments of the subject invention are not
limited thereto.
[0025] Though not shown in FIG. 1, the photovoltaic cell 100 can
further include a substrate such that the first electrode 20 is on
the substrate. In various embodiments, the substrate can be
flexible or rigid. In an alternative embodiment, no substrate is
present and the first electrode 20 can also function as a
substrate.
[0026] The first electrode 20 can include one or more of the
following materials: indium tin oxide (ITO), indium zinc oxide
(IZO), aluminum tin oxide (ATO), aluminum zinc oxide (AZO), a
transparent conducting polymer, carbon nanotubes, silver nanowire,
LiF/Al/ITO, Ag/ITO, and CsCO.sub.3/ITO. In a particular embodiment,
the first electrode 20 can be transparent to at least a portion of
visible light; for example, the first electrode 20 can be a
transparent conductive oxide (TCO), such as an ITO electrode. The
second electrode 60 can include one or more of the following
materials: ITO, IZO, ATO, AZO, silver, silver ink, silver
particles, calcium, magnesium, gold, aluminum, carbon nanotubes,
silver nanowire, LiF/Al/ITO, Ag/ITO, and CsCO.sub.3/ITO. In an
embodiment, the second electrode 60 can be transparent to at least
a portion of visible light. In a particular embodiment, the second
electrode 60 can be a silver electrode or an aluminum
electrode.
[0027] The ETL 30 can include one or more of the following
materials: a metal oxide such as titanium dioxide (TiO.sub.2) or
zinc oxide (ZnO), a metal oxide-polymer composite such as a
ZnO-polymer composite or a TiO.sub.2-polymer composite, and
polyvinylpyrollidone (PVP). For example, the ETL 30 can be a
ZnO-PVP composite, such as a ZnO-PVP nanocomposite (including
nanoparticles). In a particular embodiment, the ETL 30 can be a
ZnO-PVP nanocomposite formed by baking a coated mixture of zinc
acetate and PVP in ethanol and/or ethanolamine.
[0028] The HTL 50 can include one or more of the following
materials: molybdenum and a metal oxide such as molybdenum oxide.
For example, the HTL 50 can be thermal-evaporated molybdenum oxide
or solution-processed molybdenum oxide.
[0029] The photoactive layer 40 can include one or more of the
following materials: poly(dithienogermole)-thienopyrrolodione
(PDTG-TPD), poly(distannyl-dithienogermole)-thienopyrrolodione,
poly(dithienosilole)-thienopyrrolodione (PDTS-TPD),
(6,6)-phenyl-C71-butyric acid methyl ester (PC71BM),
poly[4,8-bis-substituted-benzo[1,2-b:4,5-b]dithiophene-2,6-diyl-alt-4-sub-
stituted-thieno[3,4-b]thiophene-2,6-diyl] (PBDTTT),
PDTG-TPD:PC71BM, PDTS-TPD:PC71BM,
poly(distannyl-dithienogermole)-thienopyrrolodione:PC71BM,
dithienogermole-thienopyrrolodione (DTG-TPD),
dithienosilole-thienopyrrolodione (DTS-TPD), and copper indium
gallium (di)selenide (CIGS). In a particular embodiment, the
photoactive layer 40 can include a polymer and a fullerene. In
another embodiment, the polymer of the photoactive layer can have a
deep highest occupied molecular orbital (HOMO) energy of more than
5.3 eV. For example, the photoactive layer 40 can be
PDTG-TPD:PC71BM or PDTS-TPD:PC71BM.
[0030] In embodiments of the subject invention, exciton generation
can occur when light is incident on the photovoltaic cell 100, for
example upon solar illumination. Electrons and holes can be
collected by the ETL 30 and the HTL 50, respectively.
[0031] In many embodiments, the ETL 30 can be UVO-treated. That is,
during fabrication of the photovoltaic cell 100, after depositing
the ETL 30, the ETL 30 can be UVO-treated before deposition of the
next layer on the ETL 30 (e.g., the photoactive layer 40). In a
particular embodiment, the ETL 30 can be a UVO-treated ZnO-PVP
nanocomposite formed by baking a coated mixture of zinc acetate and
PVP in ethanol and/or ethanolamine and then providing UVO
treatment.
[0032] Treating the ETL with UVO can result in advantageous
properties compared to an ETL that is not UVO-treated. For example,
UVO treatment can passivate the defects in the ETL, for example in
ZnO nanoparticles. In addition, in a ZnO-PVP nanocomposite ETL, UVO
treatment can help remove some or all of the PVP-rich layer from
the surface of the film, thereby exposing the ZnO nanoclusters to
the film surface. The defects in ZnO present before UVO treatment
can serve as recombination centers which result in significant
photocurrent loss. Also, the insufficient contact between the
photoactive layer and the ZnO (which occurs when the PVP-rich layer
dominates the surface of the ETL film) can retard charge
collection. Upon UVO treatment, a photovoltaic cell shows an
increase in short circuit current and fill factor. For example, a
photovoltaic cell with UVO treatment of the ETL can show an
increase of about 10% in short circuit current and an increase of
bout 5% in fill factor compared to a similar photovoltaic cell with
an ETL that has not been UVO treated.
[0033] FIG. 2a shows the current-voltage characteristics for
photovoltaic cells having a non-UVO-treated ZnO-PVP nanocomposite
ETL and a UVO-treated ZnO-PVP nanocomposite ETL (with UVO treatment
for four different lengths of time). FIG. 2b shows the external
quantum efficiency (EQE) spectra for photovoltaic cells having a
non-UVO-treated ZnO-PVP nanocomposite ETL and a 10-minute
UVO-treated ZnO-PVP nanocomposite ETL. An enhanced efficiency is
observed through the full spectral range from 350-700 nm for
photovoltaic cells with the UVO-treated ZnO-PVP nanocomposite films
compared to cells without UVO treatment.
[0034] Polymer bulk heterojunction (BHJ) photovoltaic cells, such
as those of the subject invention, are advantageous due to their
potential for low-cost energy harvesting. Donor polymers which
provide enhanced open-circuit voltage (Voc), light absorption, and
high short circuit currents when blended with fuilerenes can be
used. For example, the Voc of BHJ polymer photovoltaic cells using
low band-gap polymers such as PBDTTT can be tuned by incorporating
stronger electron-withdrawing groups onto the polymer backbone to
lower the highest occupied molecular orbital (HOMO) energy (Chen et
al., Nature Photon, 2009). Inverted PDTG-TPD:PC71BM photovoltaic
cells show higher short-circuit current density (Jsc) and fill
factor (FF) compared to devices with an analogous
polydithienosilole containing polymer (e.g., PDTS-TPD), leading to
inverted polymer photovoltaic cells with power conversion
efficiencies (PCEs) of 7.3%.
[0035] In a conventional BHJ polymer photovoltaic cell, holes are
extracted to the bottom ITO anode and electrons are extracted to
the top cathode. Poly(ethylenedioxythiophene) doped with
poly(styrene-sulfonate) (PEDOT:PSS) can be used as the anode
interlayer. However, the use of acidic PEDOT:PSS in contact with
the ITO electrode can be problematic since the ITO electrode is
etched and degraded by PEDOT:PSS during processing. To facilitate
electron extraction, a reactive low work-function metal such as
LiF/Al can be used is used as a top electrode. The use of low work
function metals as a cathode can give rise to device stability
problems in polymer photovoltaic cells due to oxidation of the
electrode when exposed to air. The inverted device geometry differs
from the conventional geometry in that the bottom electrode (e.g.,
ITO) is used as the cathode and the top electrode (e.g., a metal
electrode) is used as the anode. In an embodiment of the subject
invention, the first electrode can be an ITO cathode, and the ETL
on the ITO cathode can be a metal oxide, which can include, for
example, ZnO or TiO.sub.2. A thin transition metal oxide film, for
example molybdenum oxide can be the HTL under the second electrode,
and the second electrode can be a metal anode. The use of
transition metal oxides like molybdenum oxide along with Ag or Al
as an anode contact has an additional advantage of enhanced hole
extraction compared with the conventional PEDOT:PSS contact. One
benefit of this inverted device geometry is its compatibility with
large-scale R2R processing methods for ease of processing and
improved device stability.
[0036] In an embodiment, a photovoltaic cell can have an inverted
device geometry including a solution-processed or sputtered metal
oxide ETL. ZnO can be included in the ETL and has high electron
mobility and high optical transparency. Numerous methods can be
employed to synthesize ZnO colloidal nanoparticle (NP) films or ZnO
sol-gel processed films from various precursors. Some challenges in
fabricating ZnO NP films or ZnO sol-gel films can include poor
spatial distribution of nanoparticles and the need for surface
passivation of ZnO due to the presence of defects. These issues, if
not addressed, can lead to inconsistent device performance and low
yields in an inverted BHJ photovoltaic cell. Thus, the UVO
treatment which passivates the ZnO NP or ZnO sol-gel films without
aggregation is critical to the realization of high efficiency
inverted polymer photovoltaic cells.
[0037] In an embodiment, the ETL can be a ZnO-PVP composite sol-gel
film or a ZnO-PVP nanocomposite. The composite film can include ZnO
nanoclusters whose growth is mediated by a PVP polymeric matrix.
ZnO-PVP nanocomposite films have the following advantages over
conventional ZnO sol-gel films: (a) The ZnO nanocluster size and
its concentration can be tuned by controlling the Zn.sup.2+/PVP
ratio; (b) the distribution of ZnO nanoclusters in the PVP polymer
is uniform as compared to aggregation observed in ZnO sol-gel films
without PVP; and (c) PVP capping molecules reduce defects in ZnO
nanoclusters. PVP can passivate the ZnO nanoclusters and improve
the film-forming capability of ZnO-PVP nanocomposite films. Also,
because the sol-gel processing for the ZnO-PVP nanocomposite can be
performed in air, this approach to depositing the ZnO ETLs is
advantageously compatible with large-scale R2R processes.
[0038] In many embodiments, charge collection efficiency can be
improved in an inverted PDTG-TPD bulk heterojunction photovoltaic
cell using a ZnO-PVP nanocomposite film as the electron transport
layer to attain organic polymer photovoltaic cells with AM1.5 PCE
in excess of 8%. The use of PVP as an organic capping molecule and
polymeric matrix for ZnO can give electron-transporting
nanocomposite films with excellent film-forming characteristics.
UVO treatment can remove PVP from the surface of the film and
consequently expose the ZnO nanoclusters to the film surface. Using
a UVO-treated ZnO-PVP nanocomposite ETL, inverted PDTG-TPD (or
PDTS-TPD) photovoltaic cells can have a PCE of higher than 8%. This
is due at least in part to improved charge collection by the
nanocomposite film. This approach for fabricating highly-efficient
inverted polymer photovoltaic cells can advantageously be applied
in large-scale roll-to-roll (R2R) device fabrication.
[0039] In an embodiment, a method of fabricating a photovoltaic
cell can include: forming an ETL on a first electrode; forming a
photoactive layer on the ETL; forming an HTL on the photoactive
layer; and forming a second electrode on the HTL. The first
electrode can be a cathode, and the second electrode can be an
anode, though embodiments are not limited thereto. In a particular
embodiment, the first electrode can be a bottom cathode, and the
second electrode can be a top anode, such that when the
photovoltaic cell is in use, the photovoltaic cell has an inverted
configuration. Light can enter the photovoltaic cell such that it
goes through the first electrode, though embodiments are not
limited thereto.
[0040] In an embodiment, the method can further include providing a
substrate and forming the first electrode on the substrate. In
various embodiments, the substrate can be flexible or rigid. In an
alternative embodiment, no substrate is present and the first
electrode can also function as a substrate.
[0041] The first electrode can include one or more of the following
materials: ITO, IZO, ATO, AZO, silver, calcium, magnesium, gold,
aluminum, carbon nanotubes, silver nanowire, LiF/Al/ITO, Ag/ITO,
and CsCO.sub.3/ITO. In a particular embodiment, the first electrode
can be transparent to at least a portion of visible light; for
example, the first electrode can be a TCO, such as an ITO
electrode. The second electrode can include one or more of the
following materials: ITO, IZO, ATO, AZO, silver, calcium,
magnesium, gold, aluminum, carbon nanotubes, silver nanowire,
LiF/AUITO, Ag/ITO, and CsCO.sub.3/ITO. In a particular embodiment,
the second electrode can be a silver electrode or an aluminum
electrode.
[0042] The ETL can include one or more of the following materials:
a metal oxide such as TiO.sub.2 or ZnO, a metal oxide-polymer
composite such as a ZnO-polymer composite or a TiO.sub.2-polymer
composite, and polyvinylpyrollidone (PVP). For example, the ETL can
be a ZnO-PVP composite, such as a ZnO-PVP nanocomposite (including
nanoparticles). In an embodiment, the ETL can include a metal
oxide, and the metal oxide can be deposited by solution processing
or sputtering. In a particular embodiment, the ETL can be a ZnO-PVP
nanocomposite, and forming the ETL can include baking a coated
mixture of zinc acetate and PVP in ethanol and/or ethanolamine.
[0043] The HTL can include one or more of the following materials:
molybdenum and a metal oxide such as molybdenum oxide. In an
embodiment, forming the HTL can include thermally evaporating or
solution processing the HTL. For example, the HTL can be
thermal-evaporated molybdenum oxide or solution-processed
molybdenum oxide.
[0044] The photoactive layer can include one or more of the
following materials: PDTG-TPD,
poly(distannyl-dithienogermole)-thienopyrrolodione, PDTS-TPD,
PC71BM, PBDTTT, PDTG-TPD:PC71BM, PDTS-TPD:PC71BM,
poly(distannyl-dithienogermole)-thienopyrrolodione:PC71BM, DTG-TPD,
DTS-TPD, and CIGS. In a particular embodiment, the photoactive
layer can include a polymer and a fullerene. For example, the
photoactive layer can be PDTG-TPD:PC71BM or PDTS-TPD:PC71BM. The
photoactive layer can be deposited by, for example, sputtering,
co-evaporation, and/or solution processing.
[0045] In many embodiments, the ETL can be UVO-treated. That is,
during fabrication of the photovoltaic cell, after depositing the
ETL, the ETL can be UVO-treated before deposition of the next layer
on the ETL (e.g., the photoactive layer). In a particular
embodiment, the ETL can be a UVO-treated ZnO-PVP nanocomposite and
can be formed by baking a coated mixture of zinc acetate and PVP in
ethanol and/or ethanolamine and then providing UVO treatment.
[0046] Embodiments of the subject invention are also drawn to
methods of using a photovoltaic cell to capture light energy (e.g.,
solar energy). The photovoltaic cell can be as described
herein.
MATERIALS AND METHODS
[0047] The detailed synthesis, polymer characterization, and
photoactive layer fabrication and testing for PDTG-TPD:PC71BM and
PDTS-TPD:PC71BM were done as reported in Amb et al. (2011). PC71BM
used for OPV cell fabrication was purchased from Solenne. Polymers
and PC71BM were dissolved in chlorobenzene with 1:1.5 (8 mg mL-1:12
mg mL-1) weight ratio and 5% volume ratio of 1,8-diiodooctane (DIO)
was added as a processing additive prior to use. The ZnO-PVP
nanocomposite was prepared from a precursor, in which zinc acetate
dihydrate (Zn(CH.sub.3COO).sub.2.2H.sub.2O, Aldrich, 99.9%, 110 mg)
and polyvinylpyrollidone (PVP, 25 mg) were dissolved in 10 mL of
ethanol. Ethanolamine was added to the precursor as a stabilizer in
equal molar concentration to zinc acetate dihydrate. The ZnO-PVP
precursor was spin-coated on indium tin oxide (ITO)-coated glass
substrates, which were first cleaned with detergent, ultrasonicated
in water, acetone, and isopropyl alcohol, and subsequently dried
via N2 gun. The films were annealed at 200.degree. C. for 40
minutes in air. After annealing and slow-cooling to room
temperature, the ZnO-PVP composite films were UVO treated using a
UVO cleaner. The film thicknesses for the ZnO-PVP composite film
before and after UVO treatment were 36 nm and 33 nm, respectively.
The polymer-fullerene solutions were then spin-coated and the
resulting film with thickness of 110 nm was annealed at 80.degree.
C. for 30 minutes. Finally, thin films of MoO.sub.3 (10 nm) and Ag
(100 nm) were deposited through shadow masks via thermal
evaporation. The active area of the device was 4.6 mm.sup.2. For PV
measurements, a light mask with an area of 3.04 mm.sup.2 was used
to define the active area of the device. Device characterization
was carried out in air after encapsulation using an Air Mass 1.5
Global (A.M. 1.5G) solar simulator with irradiation intensity of
100 mW/cm.sup.2. The EQE spectra for the inverted polymer
photovoltaic cells were measured on an EQE measuring system and are
shown in FIG. 2b.
[0048] All patents, patent applications, provisional applications,
and publications referred to or cited herein (or in the references
section) are incorporated by reference in their entirety, including
all figures and tables, to the extent they are not inconsistent
with the explicit teachings of this specification.
[0049] Following are examples that illustrate procedures for
practicing the invention. These examples should not be construed as
limiting. All percentages are by weight and all solvent mixture
proportions are by volume unless otherwise noted.
Example 1
[0050] A photovoltaic cell was fabricated using ITO as the first
electrode (cathode), ZnO-PVP nanocomposite as the electron
transporting layer (ETL), PDTG-TPD:PC71BM as the photoactive layer,
molybdenum oxide as the hole transporting layer (HTL), and silver
as the second electrode (anode). Despite the advantages of improved
spatial distribution and passivation of ZnO nanoclusters, which the
addition of PVP provides to the ZnO sol-gel film, PVP is an
insulating polymer that can hinder the charge collection in the
inverted photovoltaic cells due to poor electronic coupling between
the ZnO nanoclusters within the nanocomposite and PC71BM in the
active layer. In order to ensure a good contact between the ZnO
nanoclusters and PC71BM, ultraviolet-ozone (UVO) treatment on the
ZnO-PVP nanocomposite films, before forming the photoactive layer,
to remove PVP from the surface. UVO treatment can remove PVP
without significantly altering the size, shape, or spatial
distribution of the nanoclusters films.
[0051] The photo characteristics for inverted PDTG-TPD:PC71BM
photovoltaic cells were measured with an A.M. 1.5G solar simulator
as the light source. FIG. 2a shows the current-voltage
characteristics for photovoltaic cells having a non-UVO-treated
ZnO-PVP nanocomposite ETL and a UVO-treated ZnO-PVP nanocomposite
ETL (with UVO treatment for four different lengths of time). The
inverted photovoltaic cells with the as-prepared (non-UVO-treated)
ZnO-PVP nanocomposite showed a low FF of 25.5% and Jsc of 10.9
mA/cm2. The poor FF in the cells without UVO treatment can be
attributed to the presence of PVP on the surface of the
nanocomposite film. As expected, the insulating PVP polymer hinders
the electronic coupling between the ZnO nanoclusters in the
nanocomposite film and PC71BM in the active layer, thus limiting
the charge collection of the photogenerated electrons in the cells.
The ZnO-PVP nanocomposite films were subsequently UVO treated for
5, 10, 20, and 30 minutes yielding significant enhancements in Jsc
and FF for the inverted PDTG-TPD:PC71BM photovoltaic cells compared
to cells with as-prepared nanocomposite films.
[0052] Table 1 summarizes the device performance for inverted
photovoltaic cells with, and without, the UVO-treated ZnO-PVP
nanocomposite films. UVO treating the ZnO-PVP nanocomposite films
for 10 minutes led to an optimal device with a 29.4% enhancement in
Jsc and an enhancement in the FF by 2.7 times resulting in an
average power conversion efficiency of 8.1%. The average PCE of
8.1+/-0.4% is based on the measurement results from over 100
photovoltaic cells fabricated. The optimal device had a Jsc of 14.4
mA/cm.sup.2, a Voc of 0.86 V, a FF of 68.8%, and a PCE of 8.5%. For
devices with ZnO-PVP composite films that have been UVO treated
below or above 10 minutes, a reduction in FF was observed. This
reduction in FF may be due to PVP still present on the surface of
the composite film if the UV-ozone treatment time is too short, or
excess oxygen present on the ZnO film surface which reduces the
electron extraction efficiency when the treatment time is too long.
The removal of PVP from the ZnO-PVP nanocomposite film surface by
UVO treatment greatly enhances the charge collection efficiency in
photovoltaic cells.
TABLE-US-00001 TABLE 1 Averaged photovoltaic cell performance for
inverted PDTG-TPD: PC71BM devices with as-prepared or 5, 10, 20,
30-minute UVO-treated ZnO-PVP composite ETLs. UV-ozone Average Best
treatment time J.sub.sc (mA cm.sup.-2) V.sub.oc (V) FF (%) PCE (%)
PCE (%) As-prepared 10.9 +/- 0.2 0.86 +/- 0.003 25.5 +/- 0.8 2.4
+/- 0.2 2.6 5-min. UV-ozone 13.9 +/- 0.1 0.85 +/- 0.005 56.0 +/-
3.8 6.6 +/- 0.5 7.1 10-min. UV-ozone 14.0 +/- 0.4 0.86 +/- 0.003
67.3 +/- 1.5 8.1 +/- 0.4 8.5 20-min. UV-ozone 14.1 +/- 0.2 0.86 +/-
0.003 64.8 +/- 0.8 7.8 +/- 0.2 8.0 30-min. UV-ozone 14.0 +/- 0.1
0.86 +/- 0.003 61.9 +/- 1.6 7.5 +/- 0.3 7.6
[0053] FIG. 2b shows the external quantum efficiency (EQE) spectra
for photovoltaic cells having a non-U VO-treated ZnO-PVP
nanocomposite ETL and a 10-minute UVO-treated ZnO-PVP nanocomposite
ETL. An enhanced efficiency is observed through the full spectral
range from 350-700 nm for photovoltaic cells with the UVO-treated
ZnO-PVP nanocomposite films compared to cells without UVO
treatment. The maximum EQE for the optimized inverted photovoltaic
cell with a UVO-treated nanocomposite ETL and a PDTG-TPD:PC71BM
photoactive layer was 73.6%. The Jsc value was then calculated by
integrating the EQE data with the A.M. 1.5G spectrum. The
calculated Jsc value of 14.5 mA/cm.sup.2 is in good agreement with
the measured Jsc for the inverted photovoltaic cells.
Example 2
[0054] The stability of encapsulated inverted devices with
as-prepared and UVO-treated ZnO-PVP nanocomposite films was
investigated. FIGS. 6a-6c show the changes in the Jsc (FIG. 6a), FF
(FIG. 6b), and PCE (FIG. 6c) over time for inverted PDTG-TPD:PC71BM
photovoltaic cells with either untreated or UVO-treated ZnO-PVP
nanocomposite ETLs. Devices with as-prepared nanocomposite films
show low efficiencies initially, and additional light exposure is
required to achieve optimum device performance. This light-soaking
required to obtain optimum device performance has been reported by
Krebs (Org. Elec. 10, 761-768, 2009). For example, without the UVO
treatment, the initial FF was only 25.5%, and its value increased
to 63.7% upon 10 minutes of light soaking. On the other hand,
UVO-treated devices do not require light soaking. To study the
device stability of the cell without UV-ozone treatment, the cell
was given a 10-minute light soak before subsequent PV measurements.
For the devices with UVO treatment, the PV measurements were
carried out without any light soaking at all. Referring to FIGS.
6a-6c, after light soaking, the performance of the device with
untreated ZnO-PVP nanocomposite ETL reduced significantly over
time, showing that the enhancement in Jsc and FF due to light
soaking was only temporary. In contrast, the device performance
enhancement observed for photovoltaic cells with UVO-treated
ZnO-PVP nanocomposite ETLs was stable over time. In fact, there
were no measureable changes in Jsc, FF, and PCE over a period of 1
month provided that the encapsulated devices are stored in a
nitrogen glove box. Thus, UVO treatment of the ZnO-PVP
nanocomposite films provides a permanent enhancement to the
inverted photovoltaic cells.
[0055] Encapsulated devices with UVO-treated ZnO-PVP nanocomposite
films were then sent to NEWPORT Corporation for certification. The
photo J-V characteristics and corresponding photovoltaic cell
parameters are shown in FIG. 3. A power conversion efficiency of
7.4+/-0.2% was certified for the devices. While this certified
efficiency is about 10% less than the power conversion efficiency
measured in our laboratory due to a reduction in Jsc and FF in the
certified device, the reduction in power conversion efficiency in
the certified cells can be attributed to degradation during transit
from Florida to California because of the un-optimized
encapsulation process. The devices were retested in our laboratory
after certification and we confirmed the degradation of the cells
due to encapsulation. For example, the average device performance
for the certified cells a week after certification was as follows:
Jsc=13.0 mA/cm.sup.2, Voc=0.87 V, FF=63.1%, and PCE=7.2%
Example 3
[0056] To study the nanoscale surface morphology of the as-prepared
and UV-ozone treated ZnO-PVP nanocomposite films used in our
inverted photovoltaic cells, tapping-mode atomic force microscopy
(AFM) was performed after the sol-gel films were annealed in air.
All AFM images were taken on the same substrates. FIGS. 4a and 4b
show the 3-D surface topography images for nanocomposite film
before and after UVO treatment, respectively. The ZnO-PVP
nanocomposite film shows an increase in r.m.s. roughness from 7.07
nm to 9.18 nm upon UVO treatment, suggesting that, as PVP is
removed during UVO treatment, the ZnO nanoclusters are exposed to
the surface. The phase images for the same samples are shown in
FIGS. 4c and 4d. For the nanocomposite film with no UVO treatment,
no nanoclusters were observed indicating that the surface is
covered by a thin layer of PVP. On the other hand, the phase image
for the UVO-treated ZnO-PVP nanocomposite film shows that the PVP
domain size has been reduced to 50-100 nm. Consequently, more ZnO
nanoclusters are exposed on the surface. Thus, the removal of PVP
by UVO treatment exposes the ZnO nanoclusters to the surface. The
PVP-rich and ZnO nanocluster-rich surfaces for the nanocomposite
films before and after UVO treatment are shown schematically in
FIGS. 4e and 4f, respectively.
[0057] To investigate whether the removal of PVP from the
nanocomposite film surface altered the film thickness, step-height
measurements were performed for the films before and after UVO
treatment. The average thickness of the nanocomposite film was
reduced by about 10% after 10 minutes of UV ozone treatment, from
36 nm in the as-prepared nanocomposite to 33 nm in the treated
nanocomposite film. This reduction in film thickness provides
further evidence that PVP was removed upon UVO treatment of the
ZnO-PVP nanocomposite film. To further illustrate the surface
morphology for the ZnO-PVP nanocomposite films before and after UVO
treatment, FIGS. 7a and 7b show 5 .mu.m-scale phase images. Removal
of PVP by UVO treatment significantly altered the surface
morphology for the nanocomposite film. The change in the
nanocomposite film surface morphology from being PVP-rich before
UVO treatment to ZnO NP-rich after treatment supports the premise
that the removal of PVP from the nanocomposite film by UVO
treatment provides improved charge collection in an inverted
polymer photovoltaic cells due to better electronic coupling
between the ZnO nanoclusters within the nanocomposite film and
PC71BM in the active layer.
Example 4
[0058] To investigate whether the compositional changes from the
AFM data were truly due to the removal of PVP, X-ray photoemission
spectroscopy (XPS) was performed on the ZnO-PVP nanocomposite
films. Considering the UVO time required for the nanocomposite film
to optimize the device perfoimance, some changes in the chemical
composition of ZnO might be plausible. The core level XPS spectra
for the C 1s, O 1 s, and Zn 2p were measured for the as-prepared
and 10-minute UVO-treated ZnO-PVP nanocomposite films. The binding
energies were calibrated by taking the C is peak (284.6 eV) as a
reference. The 0 is XPS spectra for as-prepared and the UVO-treated
ZnO-PVP nanocomposite films are shown in FIG. 5a. UVO treatment
increased the relative magnitude for the peak at 531.4 eV, which
corresponds to the oxygen atoms bonded to Zn in the ZnO matrix, by
about 37%. Thus the number of Zn-0 bonds in the wurtzite structure
of ZnO at the surface of the film is increased. UVO treatment also
increased the relative magnitude for the peak at about 530.0 eV,
which corresponds to (Y.sup.2 ions present in the porous ZnO
clusters, but not chemically bonded to Zn in the ZnO wurtzite
structure. FIG. 5b shows the Zn (2p3/2) XPS spectra for the
as-prepared and UVO-treated ZnO-PVP nanocomposite films. The
intensity of the peak at 1021.6 eV, which corresponds to the Zn--O
bonds, increases after UVO treatment. These results are in
agreement with the result from the O 1s XPS spectra. Based on the O
1s and Zn 2p XPS spectra, the chemical composition of ZnO
nanoclusters on the surface of the nanocomposite film have become
oxygen-rich after UVO treatment.
[0059] The atomic concentrations of C, O, and Zn for the
as-prepared and 10-minute UVO-treated ZnO-PVP nanocomposite films
based on the C 1s, O 1s, and Zn 2p XPS spectra are summarized in
FIG. 5c. The atomic concentration of carbon from the PVP in the
nanocomposite is significantly reduced by UVO treatment (from 38.2%
to 15.7%). Conversely, the atomic concentrations of oxygen and zinc
present in the nanocomposite film both increase from 28.5% and
33.3% for the untreated film to 31.6% and 52.6%, respectively, for
the treated film. The relatively smaller increase in oxygen atomic
concentration compared to Zn is due to the competition between the
increases in oxygen content coming from UVO treatment versus the
decrease in oxygen content coming from the removal of PVP. These
results strongly support the assertion that UVO treatment removes
PVP from the surface of the ZnO-PVP nanocomposite film.
Example 5
[0060] The effect of UVO treatment on optical transmission for the
as-prepared and treated ZnO-PVP nanocomposite films was considered.
FIG. 8 shows UV-visible-NIR transmission spectra for these films.
Upon UVO treatment, a 6% to 10% increase in transmission across the
entire visible spectrum is observed in the nanocomposite film. This
increase may be due to reduction of the film thickness and changes
in the effective index of refraction of the nanocomposite film upon
UVO treatment. The increase in optical transparency is less than
the enhancement observed in Jsc for inverted photovoltaic cells
with UVO-treated ZnO-PVP nanocomposite films. Therefore, while the
increase in optical transparency contributes to the Jsc
enhancement, the improved charge collection due to enhanced
electronic coupling between ZnO nanoclusters in the nanocomposite
film and PC71BM in the active layer is primarily responsible for
this enhancement.
Example 6
[0061] The behavior of UVO-treated ZnO-PVP nanocomposite films on
inverted BR1 photovoltaic cells was investigated using PDTS-TPD in
the photoactive layer. Table 2 shows the device performance for
inverted PDTS-TPD and PDTG-TPD BM photovoltaic cells with
UVO-treated ZnO-PVP nanocomposite films as electron transport
layer. The inverted PDTS-TPD:PC71BM cells show similar enhancements
in FF and Jsc compared to the PDTG-TPD cells upon UVO treatment of
the ZnO-PVP nanocomposite ETL, resulting in devices with an average
PCE of 7.6%. In both the PDTG-TPD and PDTS-TPD cells, no reduction
in Voc was observed in devices with treated nanocomposite films
despite the fact that UVO treatment oxidized the film surface.
Based on these results, this approach of UVO treating the ZnO-PVP
nanocomposite ETL works very well for this family of polymers.
TABLE-US-00002 TABLE 2 Average device performance (J.sub.sc, FF,
V.sub.oc) for PDTG-TPD and PDTS-TPD inverted photovoltaic cells
with 10-minute UVO-treated ZnO-PVP composite film as the ETL. HOMO
(eV) J.sub.sc(mA/cm.sup.2) V.sub.oc(V) FF (%) PCE (%) DTS-TPD -5.65
12.9 0.90 65.4 7.6 DTG-TPD -5.60 14.0 0.86 67.3 8.1
[0062] It should be understood that the examples and embodiments
described herein are for illustrative purposes only and that
various modifications or changes in light thereof will be suggested
to persons skilled in the art and are to be included within the
spirit and purview of this application. In addition, any elements
or limitations of any invention or embodiment thereof disclosed
herein can be combined with any and/or all other elements or
limitations (individually or in any combination) or any other
invention or embodiment thereof disclosed herein, and all such
combinations are contemplated with the scope of the invention
without limitation thereto.
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